鈥淭HE US Navy is preparing to launch an extraordinary assault on our ocean ecosystems,鈥 blares a fund-raising letter sent out last year by the Natural Resources Defense Council, an environmental group based in New York city. The weapon, readers learn, is the Navy鈥檚 newest sonar system for tracking submarines, which the letter says will expose whales 鈥渢o noise pollution at a level 200 billion times greater than that which is already known to disturb them . . . millions of marine mammals may be harmed over the long-term鈥.
It was meant to sound apocalyptic. The appeal raised such a public outcry that the Navy took the unusual step of submitting its Low Frequency Active sonar system (LFA) to an environmental impact review (This Week, 3 August 1996, p 12), which is still pending. But was the controversy grounded in fact or well-meaning hype?
A little of both. Whales would only risk deafness if they hovered within a few metres of the LFA source. It does not stand out dramatically against the other noise with which humans fill the world鈥檚 oceans, including commercial shipping traffic, seismic exploration for oil and drilling rigs. Even scientists are in on the act with their controversial Acoustic Thermometry of Ocean Climates project (ATOC) which will monitor ocean temperature by recording the time that the sound it generates takes to cross the Pacific. But even though all this noise might threaten only a few unlucky ocean-dwellers with permanent deafness, it could interfere with the breeding, feeding, or migration of many more.
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Yet scientists know very little about the effects of ocean noise. Only a handful of studies exist on marine mammals and data are even scarcer for the fish and invertebrates that occupy the bottom of the marine food chain. All the more reason for us to be cautious about adding to the ocean鈥檚 din, argue the environmentalists. 鈥淎 lot of the controversy depends on whether you want to give the animals or the noise makers the benefit of the doubt,鈥 says Peter Tyack, a marine mammal biologist at Woods Hole Oceanographic Institution in Massachusetts.
Almost all this controversy concerns sound frequencies below about 1 kilohertz, because such sounds travel much farther than higher frequencies (see, 鈥淥cean sounds鈥). As a result, much of the debate about the effect of noise has focused on baleen whales and seals, which hear well at low frequencies -unlike toothed whales and many fish. High-frequency noise pollution tends to be a local problem. If a sonar depth finder blasts out 230 decibels at a frequency of 12 kilohertz, any creature swimming immediately below would get a nasty shock. 鈥淏ut anything beyond a very local area is not going to be affected,鈥 says Christopher Fox, a marine geophysicist with the US government鈥檚 Pacific Marine Environmental Laboratory in Newport, Oregon.
Rumbling on
In contrast, the hum of heavy tankers and container ships even penetrates the open ocean, far from shipping lanes. Shipping noise is the loudest sound in the 30 to 200 hertz range, which stretches from the lowest note on a piano up into the mid-range of a cello. 鈥淚t鈥檚 a low-frequency roar, a constant rumble, kind of like if you鈥檙e sitting in a movie theatre with a good spaceship sound effect,鈥 says Chris Miller, an engineer in the coastal acoustics laboratory of the Naval Postgraduate School in Monterey, California, who has spent years listening for foreign submarines on the US Navy鈥檚 network of sonar listening posts.
As traffic has increased over the past 30 years, background noise levels in the deep ocean have risen-from roughly 75 to 85 dB in the busier waters of the northern hemisphere. While that is still only equivalent to a loud whisper on land, some of the noises that rise above this background can be very loud indeed: a magnitude 4 earthquake measures a thunderous 270 dB underwater. The LFA will generate an estimated 235 dB (though exact sound levels are a military secret). And the ATOC loudspeaker emits 195 dB.
All these sounds are diminished by distance, however, thanks to spreading of the sound waves. At the Navy listening post at Point Sur, California, 140 kilometres south of the ATOC鈥檚 speaker, for example, researchers from the University of Washington in Seattle found that its intensity was now roughly equal to background noise levels. Even when audible, the signal was faint, much like a refrigerator motor in the next room, says Bruce Howe, a physical oceanographer who led the research team. 鈥淵ou have to really be aware of when it鈥檚 turned on, and then you can pick it out of the background,鈥 says Howe.
Results like Howe鈥檚 suggest that most ocean noises are unlikely to damage the hearing of marine life, with sensitivities similar to our own, beyond the immediate vicinity of an intense sound source. But establishing how large the 鈥渋mmediate vicinity鈥 is for different marine species is problematic. Researchers can鈥檛 say for sure because they are still trying to answer the fundamental question-how intense does sound have to be before the harm starts.
鈥淭he basic answer is we haven鈥檛 a clue,鈥 says Darlene Ketten, who studies whales鈥 hearing at Harvard Medical School. Since no one has figured out how to give a hearing test to, say, a living humpback whale, Ketten has to make do with looking at the anatomy of dead whales鈥 ears. She infers how well they might hear by modelling how their inner ears respond to sound and by making comparisons with other, more easily-studied, species. Humans, for example, suffer hearing loss after prolonged exposure to 80 dB above our aerial hearing threshold, and 130 dB at the frequencies we hear best causes pain. If the ears of baleen whales were as delicate as our own, their hearing loss would start around 190 dB in water and they would suffer discomfort around 220 dB. In fact, Ketten finds, 鈥渟ome species have more robust-looking inner ears, which makes me think that they might be less fragile acoustically鈥.
But she cautions that generalisations based on baleen whale hearing thresholds are unlikely to hold true for all whales. Toothed whales, such as dolphins and killer whales, hear very poorly below 1 kHz, so low frequency noises such as commercial shipping probably don鈥檛 bother them much. But most baleen whales probably hear best at these low frequencies, and might be especially sensitive to human noises.
Evidence from other marine mammals also suggests that low frequency sounds may cause at least temporary hearing loss. Although harbour seals hear best at around 10 kHz, their hearing suffers for several hours after 20 minutes鈥 exposure to 500-Hz sounds of 130 dB (about 50 dB above their hearing threshold) according to laboratory experiments by Ron Schusterman, an animal behaviourist at the University of California at Santa Cruz.
Off the coast of Newfoundland, explosions from a nearby underwater drilling operation make humpbacks more likely to blunder into fishing nets, according to a recent study by Sean Todd of the Memorial University of Newfoundland in St John鈥檚. Todd thinks the noise, still over 140 dB at 1.8 kilometres from the source, may, slightly deafen the whales and temporarily blunt their ability to detect the net.
Yet whales are clearly able to cope with some loud noises. Their own songs often register 160 to 170 dB, and some fin whales top the charts at 200 dB. During the northern winter, the low rumbling moans of blue and fin whales are the loudest sound in the 17 to 30 Hz range, boosting background levels a hundredfold, says Howe. And Adam Frankel, a bioacoustician at Cornell University in Ithaca, New York, has seen humpback whale mothers hover just 10 metres from a calf in full song-perhaps the cetacean equivalent of a soprano letting Pavarotti bellow in her ear.
Whatever the risk of hearing damage, most biologists worry more that increases in low frequency noise might subtly affect animals鈥 behaviour, gradually eroding a population鈥檚 fitness. Southern right whale populations are increasing at 5 to 7 per cent annually, while those in the north are increasing by only 1 per cent, or less. No one knows why. But the heavier shipping traffic that makes the northern oceans louder than those in the south may well play a role, speculates Hal Whitehead, a population biologist at Dalhousie University in Halifax, Nova Scotia.
Human-made noise might prevent whales from hearing one another鈥檚 courtship calls, says Tyack. In pre-industrial times, the low-frequency range of 15 to 300 Hz in which most baleen whales sing was the quietest part of the sound spectrum, nestled between the subsonic rumblings of earthquakes and the higher pitched rattle of wind, waves and rain. The whales may have evolved to sing in this range precisely because it was so quiet, says Tyack.
Jammed frequencies
But now, with human sounds filling these frequencies and raising both local and background noise levels, the whales may have a harder time hearing and being heard. If human noise pollution prevents them from finding the best mate, or even any mate, this may be another millstone around the neck of threatened whale populations.
No one knows whether this is really a problem, says Tyack. The fact that whale calls can carry thousands of kilometres does not mean that listeners actually respond to such distant signals. If the calls are really intended only for a local audience, a decrease in their audibility at maximum range might be entirely meaningless. Moreover, says Tyack, 鈥渨e don鈥檛 know whether the animal increases its source level to compensate for the noise, just the way we all talk a little louder at a cocktail party鈥. But if human-produced noise does interfere with whales鈥 communication, then shipping-constant, ubiquitous, and covering a broad range of low frequencies-is likely to be the worst offender, says Frankel.
Noise may also have subtle effects on the movements of whales. For example, when Tyack and his colleagues played recordings of undersea drilling noise to gray whales migrating down the coast of California, the whales changed course to keep noise levels below about 120 dB. The deviation was just a few hundred metres on a migration of thousands of kilometres, and probably not important biologically, says Tyack. But Whitehead notes that if the whales tried to maintain a similar noise level in avoiding the much louder LFA transmitter in the future, their detour could take them hundreds of kilometres out of their way.
Yet, last year, Frankel found that the humpbacks tended not to avoid a simulated ATOC sound source of similar intensity. Of 11 whale pods that were within the 120-dB zone, one moved away from it, two towards it, and the other eight showed no reaction.
Much may depend on whether marine mammals grow accustomed to noises they hear often. 鈥淚 suspect-that means I鈥檓 guessing-that sounds which are more constant have less potential for disturbance than transient noises,鈥 says Frankel. 鈥淚t鈥檚 certainly my experience that you can drive [a boat] close behind a whale if you drive very smoothly. Other people come in very quickly and make a lot of gear shifts, which causes the animals to react.鈥
Although emotions and research have focused on marine mammals, other forms of marine life-on which marine mammals and humans depend for food-may be even more vulnerable, warns Lindy Weilgart, a marine mammal biologist at Dalhousie. For example, killifish, pupfish and shrimp all grow more slowly and hatch a smaller proportion of young in aquaria with sound levels just 20 to 30 dB above normal, according to American and French researchers.
In the end, only one thing seems certain about noise in the ocean: sound levels will continue to increase as world trade grows. A new, larger class of commercial ships, the first to be too big to fit through the Panama Canal, is under construction. If bigger means louder, these will also boost sound levels.
鈥淲hat alarms many of us about that is that we don鈥檛 know what effect that increased noise will have across the biota,鈥 says Roger Gentry, a wildlife biologist with the National Marine Fisheries Service in Seattle. 鈥淚f there are negative effects, we want to know about them.鈥 Both policy makers and noise makers should proceed cautiously until researchers learn more about ocean noise and its effects.
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Ocean sounds
SOUND travels as a pressure wave through fluids such as water or air-the higher the pressure, the more energy it packs and the more intense the sound.
Sound intensity (loudness) is normally expressed in decibels, a measure of the sound wave pressure relative to a standard pressure, just as the familiar Celsius scale measures temperature relative to the freezing point of water. But unlike the Celsius scale, the decibel scale is logarithmic. A rise of 3 dB represents approximately a doubling in sound intensity, 6 dB a fourfold increase, and 10 dB a tenfold increase.
For historical reasons, acoustics researchers use different zero points for airborne and waterborne sounds. You must subtract 62 dB from underwater sounds to calculate their decibel equivalent in air.
Intensity also depends on distance from the sound source. By convention, sound intensities are expressed as if they were measured just 1 metre from the source.
But as the sound wave spreads outwards, its energy is distributed over the surface of a sphere of ever increasing radius so that the sound鈥檚 intensity falls by a factor of 4 for every doubling of the range-amounting to a 60 dB decrease in the first kilometre.
In the ocean, differences in density and temperature can help to bend sound waves towards the horizontal, spreading their energy over just two dimensions, not three. This 鈥渟ound channelling鈥 helps sound carry over long distances, because its intensity now only halves when the range doubles.
As well as simple 鈥渟preading loss鈥, sound waves weaken when air or water molecules absorb their energy as heat. High-frequency sounds lose energy much more quickly than lower frequencies because fluid molecules resonate more readily at higher frequencies, absorbing energy. In the ocean, absorption is negligible for sounds below about 1 kilohertz.
